A hallmark of cancer is inactivation of cell cycle checkpoints leading to deregulated cell proliferation and genetic instability. One of the checkpoints frequently inactivated in cancer monitors progress through mitosis. Loss of this checkpoint leads to errors in segregation of the genetic material during mitosis and is clinically important, since a defective mitotic checkpoint confers increased sensitivity to mitotic stress. Indeed, many of the agents that are effective in cancer therapy, such as taxol, vincristine and others, induce mitotic stress. Several human mitotic checkpoint genes have been identified, often as homologs of yeast genes, but, except for two cases of human primary tumors these genes are to mutated in human cancer or cancer cell lines. They identified a novel human mitotic checkpoint gene, hereafter referred to as chfr. In a panel of eight human cancer cell lines, chfr mRNA and protein were undetectable in three lines and a missense inactivating mutation was identified in a fourth line. When exposed to mitotic stress, the cell lines that expressed wild-type chfr arrested in G2 or prophase, whereas the cell line that had lost chfr function passed through prophase and arrested in metaphase. Thus, unlike all previously known mitotic checkpoint genes, which regulate the transition from metaphase to anaphase, chfr defines a novel early mitotic checkpoint. Sequence analysis suggests that chfr may be a ubiquitin-protein ligase. Based on their preliminary results and the sequence information, they hypothesize that chfr is a mitotic checkpoint gene that is frequently mutated in human cancer and which exerts its checkpoint function by acting as a ubiquitin-protein ligase. They proposed the following specific aims to test this hypothesis: 1) Chfr mutation analysis in cell lines and primary tumors. 2) Use molecular markers to identify biochemical processes that are regulated, directly or indirectly, by chfr. 3) Establish whether Chfr has ubiquitin-protein ligase activity, whether its ubiquitin-protein ligase activity is required for checkpoint function and whether in vivo Chfr is a submit of a multi-protein complex. 4) Identify in vivo physiological target(s) of the ubiquitin-protein ligase activity of Chfr.